The
solubilities of anhydrite in the ternary systems CaSO<sub>4</sub> +
MSO<sub>4</sub> + H<sub>2</sub>O (M = Co, Ni) were determined
through isothermal solution saturation at 348.1 K and 363.1 K. At
low bivalent metal sulfate concentrations, anhydrite solubility decreases
until it eventually reaches a minimum. Anhydrite solubility subsequently
increases with the amount of heavy metal sulfate to a maximum. At
this point, further increases in the concentration of metal sulfate
decreases the solubility of anhydrite until saturation of the added
bivalent metal sulfate. A Pitzer thermodynamic model was selected
to predict isopiestic data including calcium sulfate solubilities
of the ternary systems CaSO<sub>4</sub> + MSO<sub>4</sub> + H<sub>2</sub>O (M = Mn, Co, Ni, Cu, Zn) from 298.1 K to 373.1 K. The functional
dependencies of the MSO<sub>4</sub> (M = Ni, Cu, Zn) ion interaction
parameters with temperature were determined, and the third virial
parameters were given. The calculated solubilities are in agreement
with the available experimental data. Using the Pitzer model and parameters,
the solubility isotherms of metastable solid-phase hemihydrate, as
well as gypsum and anhydrite, in the CaSO<sub>4</sub> + MSO<sub>4</sub> + H<sub>2</sub>O (M = Mn, Co, Ni, Cu, Zn) systems were predicted
over a wide range of temperatures and concentrations.